1. Field of the Invention
The present invention relates to a liquid reducing agent injection nozzle which is used in an exhaust emission purifying system for reducing NOx in exhaust gas using a liquid reducing agent, and particularly to a liquid reducing agent injection nozzle having a novel structure, in which accumulation of a reducing agent component of the liquid reducing agent onto a wall surface of the exhaust passage upon being injected in an exhaust flow passage, can be appreciably reduced.
2. Description of the Related Art
As conventional exhaust emission purifying systems which eliminate NOx contained in engine exhaust gas, an exhaust emission purifying device, which injects and supplies a liquid reducing agent to an exhaust gas moving in a region prevailing in the upstream of a reduction catalyst disposed on an engine exhaust system to cause catalytic reaction between NOx and the reducing agent in the exhaust gas so as to purify NOx into harmless components, is proposed (for example, see Japanese Patent Publication No. 2000-27627).
In this exhaust emission purifying device, as shown in FIG. 13, a liquid reducing agent injection nozzle 100 is protruded from the wall surface of an exhaust flow passage 102 toward an adjoining position with respect to a center of the exhaust passage and is secured on the wall of the exhaust flow passage so that a nozzle tip end portion 100a which is bent is arranged along a flow direction of the exhaust gas. Urea aqueous solution, for example, as the liquid reducing agent is spouted and supplied toward the exhaust passage wall surface from a number of nozzle holes 101 which are formed around the nozzle tip end portion 100a as shown in FIG. 14, and each of which is formed in a radial direction perpendicular to a center axis of the nozzle tip end portion 100a. As a result, the spouted liquid reducing agent is diffused and mixed with the exhaust gas by the flow of the exhaust gas, and efficiency of NOx conversion using the reduction catalyst is increased.
For the liquid reducing agent injection nozzle protruded into the exhaust passage, it is desirable that the nozzle tip end portion is disposed on an approximately center of the exhaust passage in order that the liquid reducing agent is distributed substantially uniformly in the exhaust gas. Due to a cantilever supporting structure, however, when the liquid reducing agent injection nozzle is used in particularly large-size cars, the injection nozzle might be damaged due to a vibration of vehicle body or the like, or heat capacity might become large and then a reducing agent component of the liquid reducing agent might be accumulated onto the inner wall of the injection nozzle due to exhaust heat, thereby possibly blocking up the nozzle. For this reason, at present, the nozzle tip end portion is arranged to protrude from the wall surface to a position which is approximately ⅓ of a distance between the wall surface of the exhaust passage and the passage center. As to conventional liquid reducing agent nozzles, as shown in FIG. 13, the nozzle tip end portion 100a is bent at an approximately right angle, and as shown in FIG. 14, each of the nozzle holes 101 is formed in a radial direction of the nozzle tip end portion 100a, namely, in a direction toward the passage wall surface. Since an exhaust flow rate near the passage wall is slower than that of the passage center, the diffusion and mixing of the liquid reducing agent are not sufficient, and the reducing agent component of the liquid reducing agent might be easily adhere to a wall surface on a side on which the nozzle is attached. For this reason, when a urea aqueous solution is used, for example, water is evaporated from the urea aqueous solution adhered to the wall surface of the exhaust passage due to the exhaust heat, and as shown in FIG. 13, urea is deposited and gradually accumulated on the exhaust passage wall surface near the nozzle attaching portion.